1. Field of the Invention
The invention relates to a method and to a device for continuously measuring the thickness of an insulating coating applied to a moving ferromagnetic substrate, such as a metal strip or roll.
2. Discussion of the Background
The travelling metal strip to be coated is especially a steel strip, which is bare or coated with a paramagnetic metal layer, such as a layer of zinc, tin, aluminium or alloys based on these metals, or else already coated with an insulating layer, such as a paint.
The thickness of the paramagnetic metal layer is generally between 1 and 100 .mu.m.
The rotating metal roll to be coated is, for example, a so-called "transfer" roll for transferring the coating onto any strip travelling over the said roll; the metal roll is, for example, a steel roll, bare or coated with a metal layer, such as a chromium layer; the insulating coating is applied to the roll and then transferred onto the strip (these methods are called forward roll coating or reverse roll coating).
The insulating coating to be applied is, for example, a paint or a polymer.
The insulating coating may be applied in the liquid state, for example by coating with a solution (especially in the case of a paint) or by extrusion (especially in the case of a polymer).
The insulating coating may also be applied in the solid state, for example by powder spraying or by pressing a polymer film onto the substrate.
A plant for continuously coating a strip generally comprises means for applying the coating to the strip, means for making the strip travel continuously from the upstream end to the downstream end of the plant and means for adjusting and/or regulating the thickness applied.
In the case of intermediate application to a transfer roll, the thickness applied to the transfer roll may be adjusted or regulated.
In the case of the application of paints, a coating head is normally used as application means and the plant then also includes means for drying the paint, these being placed downstream of the application means.
The drying means are regarded as means for solidifying the coating.
Thus, in the case of the application of paint to a metal strip, the strip passes through the paint application means, emerging therefrom coated with a layer of wet paint, and then passes through means for drying the applied coating, emerging therefrom coated with a layer of dry paint.
As non-contacting means for measuring the thickness of an insulating coating applied to a conducting substrate, devices which combine two types of sensor, the first sensor for measuring the distance from the device to the interface between the conducting substrate and the insulating coating and the second for measuring the distance from the device to the coated surface facing it, are known.
The thickness of the coating is then deduced from the difference between the datum given by the first sensor and the datum given by the second sensor.
This method and this device are, for example, described in French Patent Application No. 2,707,109, in European Patent Application No. 0,629,450 and in German Patent Application DE 4,007,363.
As an example of the second sensor, an optical distance measurement sensor may be used, for example one which operates by triangulation of beams, such as the one described, for example, in U.S. Pat. No. 5,355,083.
As an example of the second sensor, a capacitive sensor may also be used because the applied coating is insulating and because the substrate is conducting; the capacitive sensor must be calibrated depending on the dielectric constant of the coating at the measurement frequency used; specifically, it must generally be positioned a short distance from the coated substrate (about a few millimeters in order to have an accuracy of about 1 micrometer).
The operation of the capacitive-type sensors is based on the measurement of a capacitance formed between a plane electrode of the sensor and the metal substrate; this capacitance is formed from the layer of air separating the sensor from the coated substrate and of the insulating coating itself; this capacitance is measured by supplying an AC electric current; these known sensors are designed to deduce the distance separating the sensor from the surface of the coated substrate from this capacitance measurement or "capacitive signal".
As an example of the first sensor, an inductive sensor may be used.
The operation of inductive-type sensors is based on measuring, at some distance from the substrate, the magnetic induction and/or the eddy currents generated in the substrate by a solenoid in the sensor which is supplied with an AC electric current; these known sensors are designed to deduce the distance separating the sensor (or a reference plane associated with the sensor) from the substrate/insulating coating interface from this measurement or "inductive signal".
The response of an inductive sensor is not linear and it is important to calibrate it before it is used; for the calibration, the procedure as indicated in the already mentioned document FR 2,707,109 may be used;
an uncoated conducting substrate is introduced into the field of the device combining two sensors, PA1 the device is moved transversely to the substrate, PA1 for each movement, the sensor/substrate distance is measured using the second sensor (for example an optical or capacitive sensor) and the inductive signal, and PA1 the desired calibration function, namely distance=f(inductive signal), is thus established. PA1 .delta..congruent.120 .mu.m for pure zinc, PA1 .delta..congruent.70 .mu.m for pure aluminium, PA1 .delta..congruent.230 .mu.m for pure lead, PA1 .delta..congruent.20 .mu.m for cast steel (ferromagnetic steel); PA1 as a reminder, .delta. has a value of 700 .mu.m in the case of 304-type stainless steel (UGINE). PA1 before the coating is applied and using input measurement means which are placed upstream of the point of application of the coating and are pointed at a zone on the substrate to be coated, at least one pair of measurements of the distance to the said zone is made, PA1 after the coating has been applied and using output measurement means which are placed downstream of the point of application of the coating and pointed at approximately the same zone on the now coated substrate, at least one other pair of measurements of the distance to the said zone is made, PA1 the difference between the at least one distance datum given by the first inductive sensor and the at least one distance datum given by the second sensor is calculated, both at the input measurement means and at the output measurement means for approximately the same substrate zone, PA1 the thickness of the coating applied to the said substrate zone is deduced, to within a possible corrective factor, by difference between the difference calculated at the output measurement means and the difference calculated at the input measurement means. PA1 the said metal substrate is ferromagnetic, PA1 the input measurement means defining a fixed input reference plane P.degree..sub.ref,i approximately parallel to the said substrate in the said region of the substrate to be coated and called the incoming measurement zone Z.sub.B,i, the said pair of measurements is made before application, by measuring, at an input measurement mid-time t.sub.i, on the one hand, by inductive effect, a value d.sub.ind,i which would correspond, if the said substrate were not ferromagnetic, to the distance between the said reference plane P.degree..sub.ref,i and that metal surface or interface S.sub.met,i of the substrate which faces it in the zone Z.sub.B,i, and, on the other hand, without any contact, a value d.sub.surf,i which corresponds or would correspond, in the absence of a prior insulating coating on the surface of the substrate to be coated, to the distance between the said reference plane P.degree..sub.ref,i and that surface S.sub.met,i of the substrate which faces it in the zone Z.sub.B,i, PA1 the output measurement means defining a fixed output reference plane P.degree..sub.ref,o approximately parallel to the said substrate in the said now-coated zone of the substrate and called the outgoing measurement zone Z.sub.B,o, the said pair of measurements is made after application, by measuring, at a mid-time t.sub.o delayed from the time t.sub.i by a delay time T.sub.d, in the same way as previously, a value d.sub.ind,o by inductive effect and a value d.sub.surf,o on the coated zone Z.sub.B,o of the substrate, PA1 the transit time for the moving substrate to move between the input measurement means and the output measurement means through means for applying said coating being equal to T.sub.t, T.sub.d is chosen to be equal to T.sub.t so that the zones Z.sub.b,i and Z.sub.B,o correspond to the same substrate zone Z.sub.B before and after coating, PA1 and the average thickness E.sub.B of the coating applied to Zone Z.sub.B is then calculated and deduced, to within a possible corrective factor, using the formula: EQU E.sub.B ={[d.sub.ind,o -d.sub.surf,o ]-[d.sub.ind,i -d.sub.surf,i ]}. PA1 the said metal substrate is ferromagnetic, PA1 the input measurement means defining a fixed input reference plane P.degree..sub.ref,i approximately parallel to the said substrate in the said zone of the substrate to be coated and called the incoming measurement zone Z.sub.B,i, the said pair of measurements is made before application, by measuring, at an input measurement mid-time t.sub.i, on the one hand, by inductive effect, a series of N values d.sub.ind,i which would correspond, if the said substrate were not ferromagnetic, to the distance between the reference plane P.degree..sub.ref,i and that metal surface or interface S.sub.met,i of the substrate which faces it in the corresponding zones Z.sub.B,i, and, on the other hand, without any contact, a series of N values d.sub.surf,i which correspond or would correspond, in the absence of a prior insulating coating on the surface of the substrate to be coated, to the distance between the said reference plane P.degree..sub.ref,i and that surface S.sub.met,i of the substrate which faces it in the corresponding zones Z.sub.B,i, the output measurement means defining a fixed output reference plane P.degree..sub.ref,o approximately parallel to the said substrate in the said now-coated zone of the substrate and called the outgoing measurement zone Z.sub.B,S, the said pair of measurements is made after application, by measuring, at a mid-time t.sub.o delayed from the time t.sub.i by a delay time T.sub.d, in the same way as previously, a series of N values d.sub.ind,o by inductive effect and a series of N values d.sub.surf,o on the corresponding coated-substrate zones Z.sub.B,o, PA1 the values d.sub.ind,i, d.sub.surf,i, d.sub.ind,o and d.sub.surf,o denoting the averages of the respective values d.sub.ind,i, d.sub.surf,i, d.sub.ind,o and d.sub.surf,o in each series of N measurements, the transit time for the moving substrate to move between the input measurement means and the output measurement means through means for applying the said coating being equal to T.sub.t, T.sub.d is chosen to be sufficiently close to T.sub.t that the series of N zones Z.sub.B,i and the series of N zones Z.sub.B,o have at least 90% of successive substrate zones Z.sub.B in common before and after coating and thus correspond to approximately the same substrate zone before and after coating, the said successive substrate zones Z.sub.B forming a strand segment of substrate, PA1 and the average thickness E.sub.B of the coating deposited on the said strand segment of substrate is then calculated and deduced, to within a possible corrective factor, using the formula: EQU E.sub.B {[d.sub.ind,o -d.sub.surf,o ]-[d.sub.ind,i -d.sub.surf,i ]}. PA1 .tau. N, .upsilon. or T.sub.t are chosen so as to satisfy the relationship: .tau.&lt;T.sub.t /10; PA1 the values d.sub.surf,i and d.sub.surf,o are measured by capacitive effect and, the said coating to be applied having a predetermined relative dielectric constant .epsilon..sub.app1, the corrective factor [.epsilon..sub.app1 /(.epsilon..sub.app1 -1)] is applied to the calculated thickness E.sub.B of the coating; PA1 the d.sub.surf,i and d.sub.surf,o values are measured by triangulation of light beams, especially LASER beam. PA1 when the said coating is applied in the liquid or pasty state and then solidified after application, the said measurements made after application are made before solidification; PA1 the moving metal substrate is made of ferromagnetic steel; PA1 the frequency of the inductive-effect measurement results in a standard skin depth in the said steel of less than 100 .mu.m; PA1 the said steel is coated with a paramagnetic metal layer, especially a layer of zinc, aluminium, chromium, tin or their alloys; PA1 the frequency of the inductive-effect measurement causes a standard skin depth in the said layer of greater than the thickness of the said layer; PA1 the moving substrate is a travelling strip; and PA1 the moving substrate is a rotating roll. PA1 measurement means which can be pointed at a measurement zone of the said travel path, in which means two distance measurement sensors are combined, the first sensor operating by inductive effect in order to measure the distance to a homogeneous non-ferromagnetic metal surface or interface placed in the said measurement zone, the second sensor being adapted for measuring the distance to a surface placed opposite the said sensor in the said measurement zone, the said measurement means being placed so as to face the same strand of the path along which the substrate travels, one of them on the input side upstream of the application means, and the other on the output side downstream of the application means, PA1 means for triggering the output measurement means after a delay of a time interval T.sub.d with respect to the time at which the input measurement means were triggered approximately equal to the transit time T.sub.t for the substrate to move between the measurement zone of the input measurement means and the measurement zone of the output measurement means, PA1 and means for calculating the difference between the at least one distance datum given by the inductive sensor and the at least one distance datum given by the second sensor, both at the input measurement means and at the output measurement means, these means being triggered after the delay time T.sub.d, and for deducing the thickness of the coating applied to the substrate zone travelling through the said output measurement zone, to within a possible corrective factor, by difference between the difference calculated at the output measurement means and the difference calculated at the input measurement means. PA1 the said second distance measurement sensor operates by capacitive effect; PA1 the said second distance measurement sensor operates by triangulation of optical beams, especially LASER beam; PA1 the application means are designed to apply a coating in the liquid or pasty state, the plant comprises means for solidifying the said coating, these being placed in the said travel path downstream of the said application means, and the output measurement means are positioned between the said application means and the said solidification means; PA1 the coating plant furthermore includes means for regulating the thickness applied by the said application means, these being designed to act continuously as a function of the coating thickness measurement delivered by the said thickness measurement device.
The response of the inductive sensor is thus "linearized" in such a way that the datum given by the sensor is a distance value.
In order to measure, without any contact, the thickness of an insulating coating on a conducting substrate, the inductive measurement method proves, however, to be unusable in very many practical cases, especially when an accuracy of about ten micrometers, or even one micrometer, is required, especially when the metal substrate is ferromagnetic and/or has a surface which is heterogeneous, something which is the case, for example, with galvanized or tin-plated steel strip or with chromium-plated steel rolls.
This is because, when the metal substrate is ferromagnetic and/or has a surface which is heterogeneous, the Applicant has observed that the large fluctuations in the electrical conductivity and in the magnetic susceptibility over the depth near the metal surface in the zone of penetration of the radiation emitted by the inductive sensor caused considerable perturbations in the "inductive signal".
The depth of the zone of penetration of the radiation emitted by the sensor depends on the frequency--this is the well-known "skin effect" phenomenon.
The standard skin depth .delta. (in mm) is calculated as being the depth in the substrate at which the intensity of the eddy currents has dropped by 37% with respect to the value at the surface; it is expressed in the following manner: ##EQU1## where .rho. is the resistivity of the substrate (.mu..OMEGA..multidot.cm), f is the measurement frequency (Hz) and .mu..sub.r the relative permeability of the substrate.
Thus, at for f=1 MHz and at room temperature:
It is therefore observed that, at 1 MHz, the standard skin depth for ferromagnetic steel and for cast steel is so small that the inductive signal is very sensitive to the slightest heterogeneities in the surface of the steel.
Thus, if the substrate consists of a ferromagnetic conducting material, such as bare steel, the signal delivered by the inductive sensor may be highly perturbed when the frequency is too high, that is to say when the standard skin depth does not reach at least 100 .mu.m.
It is also observed that, at 1 MHz, the standard skin depth for a ferromagnetic conductive substrate having a paramagnetic metal layer on the surface of a thickness generally less than 100 .mu.m is greater than the thickness of the metal layer on the surface of the substrate and the inductive signal is sensitive both to the heterogeneities in the paramagnetic metal layer and to those at the steel/metal layer interface.
Thus, if the substrate is made of a ferromagnetic conducting material, such as steel, coated with a non-ferromagnetic metal layer having a very different skin effect, the signal delivered by the inductive sensor may be highly perturbed while the standard skin depth remains greater than the thickness of the metal layer; in order for the skin depth to be less than the thickness of the metal layer, it would be necessary, conversely, to carry out the measurements at much higher frequencies; however, inductive sensors operating at such frequencies are much more expensive.
Despite an accurate prior calibration of the inductive sensor, for example, such as the one described in the document FR 2,707,109 already mentioned, the thickness datum delivered by the double-sensor device may therefore fluctuate by very large amounts that bear no relationship to the actual thickness of the insulating coating.
The Applicant has encountered this problem when carrying out measurement tests using a device which combines two sensors--an inductive sensor and a capacitive sensor.
The device used operates at a frequency of 1 MHz and includes algorithms for linearizing the response of the two sensors in the distance measurement range in question so that the difference between the linearized (or "calibrated") data from the two sensors pointed at a conducting substrate should correspond exactly to the thickness of an insulating coating applied to this substrate; this difference should be zero or close to zero if this substrate is not coated with an insulating layer.
A first series of tests consists in making a coating thickness measurement on bare metal strips, i.e. those which are not coated with an insulating layer; twenty-two equidistant reference marks, 5 cm apart, are made on the strip and an insulating coating thickness measurement is made each time a reference mark passes into the field of the sensors of the device; the difference between the linearized data from the two sensors should, within the inaccuracies of the sensors of the device, be zero for each reference mark since the strips tested are bare.
Plotted in FIG. 5 is the datum (in .mu.m) given by the device, or insulating coating "thickness", as a function of the number of the reference mark on the strip for 3 types of bare metal strip, namely a stainless steel strip ("Stainless"; points in the form of triangles), a cold-rolled carbon steel ("Cold rolled"; points in the form of diamonds) and the same steel strip galvanized ("Galvanized" with a zinc thickness of approximately 15 .mu.m; points in the form of squares).
It is found that the two-sensor device gives a datum which is close to zero and quite reproducible on a non-ferromagnetic stainless steel strip; the variation in the data over the reference marks does not exceed 2 .mu.m.
A similar result would be expected on the other two types of strip.
However, for the cold-rolled carbon steel and even more for the carbon steel coated with a conducting layer, especially a metal layer, the variation in the datum given by the device reaches 8 .mu.m in the first case and 18 .mu.m in the second case; this clearly shows that in both these cases the data given by the two-sensor device can no longer be regarded as reliable, even though these sensors are nevertheless calibrated and "linearized".
Next, ferromagnetic steel sheet specimens coated with layers of zinc of varying thickness were prepared; plotted in FIG. 6 is the datum given by the same device as a function of the actual thickness (in .mu.m) of zinc deposited on the steel sheet; it may be seen that the response of the device, the two sensors of which are nevertheless calibrated, is far from the actual insulating coating thickness value (in this case: zero) as the zinc thickness decreases; since the curve is very steep for a ferromagnetic steel substrate coated with an approximately 10 .mu.m layer of zinc, it will be appreciated that the measurement of the thickness of an insulating coating on such a substrate will be very sensitive to the slightest fluctuations in the thickness of the subjacent metal coating, as illustrated in FIG. 7.
FIG. 7 shows the data given by the same device applied, on the one hand, to a bare chromium-plated steel roll and, on the other hand, on the same chromium-plated steel roll coated with an insulating paint layer; it may be seen that any fluctuations in the thickness of the paint layer are completely masked by the very high sensitivity of the device to the slightest fluctuations in thickness, in structure or in composition of the subjacent chromium layer.
The subject of the invention is therefore a method for accurately measuring, without any contact, the thickness of an insulating coating applied to a moving ferromagnetic conducting substrate, especially a steel sheet or roll which is bare or coated with a non-ferromagnetic metal layer, using two-sensor devices, such as those described in French Patent Application No. 2,707,109, in European Patent Application No. 0,629,450 or in German Patent Application DE 4,007,363.
Apart from the question of the coating thickness measurement itself, in order to continuously regulate the thickness of a coating while it is being applied, use is made, in addition to the coating thickness measurement means, placed downstream of the application means, of an actuator for actuating the application means which is designed to vary the thickness of the coating depending on a control signal and on the electronic means for slaving the said actuator to the said measurement means depending on a set point signal corresponding to a predetermined desired thickness of coating.
When the coating is applied in the liquid state, the plant includes means for solidifying the coating downstream of the application means and of the conventional thickness measurement means, which conventional thickness measurement means, if they can only be used effectively or easily on solid coatings, must be placed further downstream of the solidification means.
The thickness-regulating means then do not allow the thickness to be regulated with a sufficiently short response time because the thickness measurement means are too far away from the coating application means.